The invention relates to an MR method of determining local relaxation time values in an examination volume.
Moreover, the invention relates to an MR imaging device for carrying out the method and to a computer program for such an MR imaging device.
In MR imaging, as is known, nuclear magnetization within the examination volume of the MR imaging device used is located by means of temporally variable, spatially inhomogeneous magnetic fields (magnetic field gradients). The MR signals used for image reconstruction are usually recorded as a voltage, which is induced in a high-frequency coil arranged in the region of the examination volume, under the effect of a suitable sequence of switched magnetic field gradients and high-frequency pulses in the time domain. The actual image reconstruction from the recorded MR signals usually takes place by Fourier transformation of the time signals. The scanning of the spatial frequency area (so-called “k-space”) assigned to the examination volume, by means of which the field of view (FOV) to be imaged and the image resolution are determined, is defined by the number, the temporal spacing, the duration and the strength of the magnetic field gradients and high-frequency pulses used. The number of phase coding steps during scanning of the k-space and thus at the same time the duration of the imaging sequence are defined as a function of the respective requirements in terms of FOV and image resolution.
From the prior art, MR imaging methods are known in which the determination of the local longitudinal and/or transverse relaxation times of the nuclear magnetization (T1-, T2- or T2*-relaxation) is of particular importance. The visualization and also the quantitative determination of the spatial distribution of the relaxation times are important for example when contrast agents which affect the relaxation of the nuclear magnetization are used in the MR imaging. Such contrast agents, which are based for example on gadolinium or on iron oxide, have recently been used also to track marked cells by means of MR and to locate active substances within the examination volume. The spatially resolved determination of relaxation times is also useful in functional MR imaging (fMRI). On the one hand, it is known from the prior art to record T1-, T2- or T2*-weighted MR images in order to visualize the spatial distribution of the relaxation times. On the other hand, for some applications, it is desirable to be able to determine the local relaxation times as accurately as possible in quantitative terms. This is the case for example in perfusion studies in which the temporal progress of the passage of a contrast agent bolus through a specific anatomical structure is studied. Another example is the measurement of the dimensions of capillary vessels and the density thereof by means of MR. Quantitative MR relaxometry can also be used for the quantitative determination of the iron content in certain internal organs (e.g. liver, lungs, brain).
One problem with MR relaxometry is that, for the spatially resolved determination of relaxation time values using conventional methods, the image recording times are undesirably long. This is due to the fact that, in conventional MR methods, usually a large number of separate MR images are recorded, specifically with different time parameter sets of the imaging sequence used. For each individual image point, the time response of the corresponding image values can then be evaluated as a function of the time parameters of the imaging sequence, such as the repetition time or the echo times for example. It is usual to determine a relaxation time value for each image point by means of a fit procedure, wherein a corresponding exponential decay function is adapted to the image values recorded in a time-dependent manner. It is also known to record T1- and T2-weighted MR images for different time parameter sets at the same time. From the MR images reconstructed for the different time parameter sets, the local relaxation time values are then adapted to multiexponential fit functions. In this method, in many cases the accuracy of determination of the local relaxation time values is unsatisfactory. In any case, in the conventional MR methods, in order to determine the relaxation time values, complete MR images for 5 to 10 different time parameter sets have to be recorded in order that a sufficiently large number of image values for different repetition or echo times exist for the fit procedure for each image point. If T1 and T2 values are to be determined at the same time, it is usually necessary, in order to achieve sufficient accuracy, to use a greater number of different time parameter sets. The image recording time is very long as a result.